Instruction Manual for Water Brake-Type Absorption Dynamometer

The
power absorbed by the Hydra-Brake® is due to a combination of
shearing and pumping action and acceleration and deceleration of the
water as it is thrown from the holes in the rotors to the holes in
the stators in passing through the brake. The major part of the
absorption is due to the acceleration and deceleration action.

For
any given speed, the power absorbed by the brake may be increased or
decreased by increasing or decreasing the depth of immersion of the
rotors in the water. At any given speed, the power absorption can be
varied from the maximum to about 2% of maximum under ideal conditions
using a continuous flow system. Our normal recommendation is to
restrict the operation of the brake between 10% and 100% of maximum
absorption capacity. However, as the skill of the operator increases,
operation under 10% of maximum can be realized if proper valves and
controls are used. See schematic of control systems for typical
installation.

A summary of requirements for good control on continuous flow systems follows:

Properly
sized brake

Constant
pressure water supply source

Properly
sized inlet and discharge valves. Use V-ported (or equivalent)
valves, which have no movement of plug with respect to seat once set
in a given position.

Atmospheric
vent for water compartment

Atmospheric
drain on discharge of brake

The
discharge line should be run downhill from the brake and contain no
U-bends or traps

Good
speed control on the prime mover

In
general, the Hydra-Brakes® are provided with ball bearings with
provisions for oil or grease lubrication. Where space is limited, and
speeds are not excessive, grease-packed cartridge bearings may be
used. (Special small, grease packed cartridge bearings are being used
at speeds of up to 26,000 rpm, but they require more frequent
attention.)

Brakes
are provided with 1/8" NPT for installation of spring-loaded
thermocouples for measuring bearing temperatures. Thermocouples and
temperature indicators are optional and can be supplied at extra
cost.

The
power absorbed by the Hydra-Brake® may be measured by two basic
methods. The first method is to measure heat rejection to the water
by using the water flow and temperature rise through the brake. This
method is used for direct mounted (uncradled) units where accuracies
in the order of 2% are satisfactory. The second and more accurate
method is the conventional method for measuring torque and speed. In
this case, the brake is mounted on trunnion bearings, and an arm is
provided for measuring torque. The cradle for this brake may be of
the cantilever type. The torque may be measured by using a weight
scale, dead weights, or by load cell of the pneumatic, hydraulic, or
strain gage type. The selection depends on the preference of the
user. Instrumentation should be selected for the degree of accuracy
required.

A
third method of the cantilever type which is gaining acceptance where
space is limited is the use of a special torque-measuring device
installed between the brake and the prime mover. This unit consists
essentially of a series of axial beams instrumented with strain
gages. Flanges are attached on both ends of these beams, one for
mounting onto the prime mover and the other end to support the brake.
The signal for the strain gages is fed into an instrument which is
calibrated to read torque.

Casing,
rotors, and stators are made of aluminum when light weight is
important, or stainless steel, which costs and weighs more but is
superior to aluminum for its wear and anticorrosion properties.

Most
models are furnished with a splined brake shaft for ease of mounting
and to permit some misalignment. A splined adapter shaft is used
between the prime mover and the Hydra-Brake® shaft.

There
are two general systems used for supplying water to the brakes. The
first and most widely used is the continuous flow system described in
detail below. This system is satisfactory for testing turbines and
gearboxes. The other is the closed loop system with heat exchangers.

In
order to ensure proper operation of the Hydra-Brake®, a constant
pressure supply source must be used. Some methods for obtaining this
supply follows:

A
simple method is to use an open tank mounted at sufficient height to
provide ample pressure to the brake. A liquid level control and an
overflow pipe should be incorporated in this tank.

When
sufficient height is not available a centrifugal pump may be used to
supply the brake. The pump suction line should be immersed in the
tank or reservoir of constant level. An air chamber should be used on
the discharge line as close as possible to the pump.

A
pressure-regulating valve which is fed from a high-pressure system
may be used to supply water to the brake at reduced pressure. It is
recommended that a surge chamber is installed after the valve to
dampen out any flow pulsation.

In
rare instances, the plant water supply system may be satisfactory
without further regulation.

If
more than one brake is to be fed from a constant pressure supply
source a manifold may be used. However, the supply line and manifold
should be oversized that changing the flow to one or more brakes does
not require readjustment of the other brakes on the line.

All
hand valves should be properly sized V ported or equivalent with such
construction that there is no movement of the valve plug with respect
to the valve seat once set in a given position. When sizing inlet
valves assume the pressure at the inlet to the brake is approximately
10 psi. The discharge pressure of the brakes varies with size, power
absorbed, and speed. It is for these reasons that definite sizing
information is not available. In the past, when actual test data has
been lacking, we assumed a discharge pressure of 10 psi for low speed
and power and 30 psi for high speed and power.

Motorized
valves may be used for remote or controller actuation. It is
recommended that these valves be initially oversized with reduced
ports used to suit actual requirements. Valves selected should be of
such design that there is no movement of the valve plug with respect
to the valve seat once set in a given position. Valves with any
backlash in actuating linkage should not be used. If possible, manual
inlet valves should be installed in the control room in lieu of
remote control valves.

A
flowmeter should be used in all installations and located in the
control room if possible. When measuring power by heat rejection to
water, a 600 mm scale should be used, for other installations, a 250
mm scale is satisfactory. Locate the flowmeter upstream of the inlet
valve to prevent air bubbles from forming in the meter. The maximum
supply pressure will be governed by the allowable pressure on the
flowmeter. When required, remote reading flowmeters may be used.

A
water compartment vent should be provided on all installations. When
measuring power by torque and speed, a vent to atmospheric drain with
a hose running downhill with no U-bends or traps should be used.

Another
method for the above and the method to be followed on all other
installations is the use of a float or swing check valve mounted
above the brake. In any case, should the inlet pressure to the brake
fall much below atmospheric (say, 5" Hg vacuum), provide
additional venting by connecting check valves to the unused vent
ports if available, in one of the inlet ports if available, or in the
inlet lines as close to the brake as possible.

A
temperature pickup (e.g. thermocouple) on the inlet line is necessary
only when measuring power by heat rejection to water. A thermocouple
or equivalent should always be used on the discharge line. The
temperature sensing element should be corrosion-resistant to water.
High-accuracy thermometers may be used if the operator is to be
stationed at the text location. In all cases, the temperature-sensing
element should be located some distance from the brake in the center
of the water line.

A
Hydra-Brake® should be properly selected for the power and speed
range of the test. The brake will operate satisfactorily under ideal
conditions as low as 2% of rated power. However, it is generally our
recommendation that, should the power absorbed drop below 10% of
capacity, then discs should be removed to improve control of the
brake. Information on removal is given in the "Assembly and
Disassembly Instructions."

A
flexible hose on the inlet and discharge connections to the brake
should be used for all installations.

When
frequent removal of the brake is anticipated, the use of quick
disconnect couplings on the inlet and discharge lines is suggested.